A zirconia sintered body containing a small amount of Y2O3 in solution as a stabilizer (hereinafter referred to as Y-TZP) is in extensive use as mechanical/structural materials, e.g., cutting tools, dies, nozzles, and bearings, and biomaterials such as dental materials because of the high strength and high toughness thereof. In the case of dental materials, not only mechanical properties including high strength and high toughness are required but also optical properties including translucency and color tone are required from an aesthetic standpoint.
Investigations directed only to mechanical properties, i.e., strength enhancement, of Y-TZP have hitherto been made.
The mechanism of strength enhancement in Y-TZP is based on the martensitic transition of the tetragonal-phase zirconia contained in the sintered body to a monoclinic phase by stress. In ordinary-pressure sintering, which is a general process for producing Y-TZP, large voids remain in the sintered body. The fracture strength of such a sintered body is influenced by the size of large voids, and the sintered body obtained by ordinary-pressure sintering and containing large voids remaining therein has a three-point bending strength of about 1,200 MPa. In order to forcedly eliminate such voids, investigations are being made on strength enhancement with a hot isostatic press (hereinafter abbreviated to HIP) or a hot press.
Non-patent document 1 has reported the three-point bending strength of a zirconia sintered body having an yttria concentration of 2-3 mol % produced by the HIP pressure sintering method. This sintered body has a higher strength than the sintered body obtained by ordinary-pressure sintering. However, the sintered body produced by the HIP pressure sintering method has an average strength of about 1,700 MPa at the most and has insufficient translucency.
Patent documents 1 to 3 disclose that a composite sintered body composed of zirconia having an yttria concentration of 2-4 mol % and an oxide, such as alumina, and produced by the pressure sintering method using an HIP or the like has a strength as high as 2,000 MPa or above. Zirconia having an yttria concentration of 2-4 mol % and containing no oxide such as alumina is shown therein as a Comparative Example. However, this comparative sintered body has an average strength as low as 1,650 MPa and has insufficient translucency, like the sintered body of non-patent document 1.
Patent document 4 discloses, like patent documents 1 to 3, that a composite sintered body composed of zirconia having an yttria concentration of 2-4 mol % and alumina has a strength as high as 2,000 MPa or above. As a Comparative Example zirconia having an yttria concentration of 2-4 mol % and containing no oxide such as alumina is shown therein which has an average strength of 1,854 MPa at the most. However, this sintered body has insufficient translucency like the sintered body of non-patent document 1.
For obtaining a sintered body having both translucency and color tone, there is a method in which a black sintered body obtained with an argon HIP is oxidized by heating in the air (see non-patent document 2). However, there is a problem that the re-oxidation results in a decrease in sintered body strength.
On the other hand, orthodontic brackets made of zirconia containing yttria have been disclosed as an article having some degree of translucency (see patent documents 5 to 7). These brackets each are produced with an oxygen-containing-gas HIP and are insufficient in strength and transmittance.
Hitherto, no Y-TZP has been obtained which combines a strength as high as 1,700 MPa or above with translucency and color tone, which are required from an aesthetic standpoint. It has been impossible to obtain a sintered body having a strength of 1,900 MPa or higher from zirconia having an yttria concentration of 2-4 mol % without combining the zirconia with an oxide such as alumina used in an amount of 1 wt % or larger, practically 10 wt % or larger.
On the other hand, among zirconia sintered bodies having especially high translucency, there is single-crystal cubic zirconia for use as an artificial gem, etc. Cubic zirconia is being produced by the so-called skull melting method, which includes placing a powder of zirconium oxide in a crucible, adding 10-20% Y2O3 thereto as a stabilizer, and then melting the zirconium oxide by high-frequency heating (see, for example, patent document 8).
However, the cubic zirconia is single-crystalline and is produced through heating at a temperature of 2,000° C. or higher, which exceeds the melting point of zirconia. The translucent zirconia obtained as a sintered body must be subjected to cutting in order to impart a desired shape thereto. The skull melting method has hence been unsuitable for use as an industrial process.
With respect to a polycrystalline transparent zirconia sintered body, a process for producing the sintered body is known in which a mixture obtained by adding 8-10% or more yttria and 5-10% or more TiO2 to zirconia is treated with a hot isostatic press (HIP) at a high temperature of 1,500° C. or above (see, for example, patent document 9). The patent document discloses that such a sintered body can be a transparent sintered body which has high translucency and has an in-line transmittance exceeding 40%.
However, such a zirconia sintered body to which enhanced translucency have been imparted by adding different elements has a bending strength as low as about 200 MPa and has hence been unusable in applications where high strength is required.
As a result of recent progress in the technology of precision processing using a CAD/CAM system, a process has come to be employed in which an artificial tooth is produced from a zirconia sintered body block. For example, a lowly sintered Y-TZP block is ground into a desired artificial-tooth shape by the system and then burned at a temperature of about 1,300° C. to obtain a sintered body having a high density. The sintered body thus produced has a problem that it has low translucency although high in strength. Use of this sintered body has hence been limited to molar teeth, which are not especially required to have aesthetic properties. As materials having higher translucency for use as, e.g., fore-teeth, glass ceramics are mainly used, such as leucite and lithium silicate. These glass ceramics have a contrast ratio, which is an index to translucency, as low as about 60% and have attained aesthetic properties akin to those of natural teeth from the standpoint of translucency. However, those glass ceramics have poor strength. Specifically, the glass ceramics have a bending strength as low as 100-300 MPa and a fracture toughness as low as about 1.0-3.0 MPa·m0.5. There has been a problem that the glass ceramics, when used as a dental material, have low mechanical strength and are apt to chip or crack during use.
A zirconia sintered body for use as a dental material, such as an artificial tooth or an orthodontic bracket, must have not only high strength but also aesthetic properties based on translucency. The zirconia sintered bodies which have been reported are ones which have low strength or ones which have high strength but do not sufficiently reconcile the strength and translucency concerning an aesthetic standpoint.
Especially in applications to orthodontic brackets among dental materials, there is a desire for a sintered body having higher strength and excellent translucency.    Patent Document 1: JP-A-60-86073    Patent Document 2: JP-A-60-226457    Patent Document 3: JP-A-60-235762    Patent Document 4: JP-A-3-80153    Patent Document 5: JP-A-3-170148    Patent Document 6: JP-A-08-117248    Patent Document 7: JP-A-11-276504    Patent Document 8: JP-A-06-172031    Patent Document 9: JP-A-62-91467    Non-Patent Document 1: Ceramics Bulletin, Vol. 64, p. 310 (1985)    Non-Patent Document 2: Shigeyuki Somiya and Masahiro Yoshimura e.d., Zirconia Ceramics 8, Uchida Rokakuho, pp. 33-43 (1986)